Monocrystalline semiconductor ingots are commonly grown from molten semiconductor material (e.g., polycrystalline silicon) held in a crucible. In the Czochralski process, for example, a charge of semiconductor material is placed in a crucible and melted by a heater surrounding the crucible. A seed crystal is brought into contact with the upper surface of the melt. The molten semiconductor material solidifies at the crystal/melt interface, joining the lattice started by the seed crystal. As the molten semiconductor material is incorporated into the growing ingot, a crystal puller slowly raises the ingot to keep the crystal/melt interface at the upper surface of the melt. Typically, the crucible is also raised during the process to keep the upper surface of the melt at a substantially constant level despite incorporation of material (from the melt into the ingot. Thus, the seed crystal gradually grows into a monocrystalline ingot, the size, shape and other characteristics of which can be controlled by controlling the pull rate, melt temperature, and other variables affecting crystal growth.
Crucibles used to grow semiconductor ingots are commonly made of an amorphous form of silica known as vitreous silica (i.e., fused quartz). Vitreous silica is favored because of its purity, temperature stability and chemical resistance. One disadvantage associated with vitreous silica crucibles is that they lose structural integrity when subjected to the high temperatures of the crystal growing process. In general, these crucibles soften with increasing temperature and are soft enough to easily flow under an applied stress when the crucible wall temperature exceeds about 1500° C. Thus, there is a risk that a vitreous crucible will buckle at the sidewalls or otherwise deform before an ingot can be completely grown. Deformation occurs most often during remelt of an imperfect crystal or melting of bead polycrystalline silicon (i.e., granular polycrystalline silicon formed in a fluidized bed).
To meet the demand for larger semiconductor wafers, crystal growers have grown larger ingots. Silicon ingots grown by the Czochralski method can now be more than 300 mm in diameter and up to a meter or more in length. If the trend continues, ingots grown in the future will be even larger. Processes for growing larger ingots can be even more demanding on crucibles than processes for growing smaller ingots. Growing a larger ingot generally requires melting a larger charge of semiconductor material in a larger crucible. The additional weight of the crucible and the larger forces applied to the crucible sidewall by the larger melt subject the crucible to higher stresses. Because more heat is required to melt the larger charge of semiconductor material and to maintain the desired melt temperature throughout the process of growing a larger ingot, the crucible may also need to be subjected to higher temperatures. Moreover, it takes longer to grow a larger ingot, which means the crucible has to withstand the stresses and heat longer.
Graphite susceptors are used to support vitreous silica crucibles because at high temperatures graphite is more resilient than vitreous silica. Graphite does not flow at typical crystal growth temperatures and thus is an adequate support for the fused silica. For instance, one common susceptor 9 includes a base 11 and two semi-cylindrical supports 13. (See FIGS. 1 & 2). A retainer 17 on the base 11 holds the lower ends of the supports 13 together in abutting relation. The seams between the supports 13 are known as susceptor splits 19.
Although graphite susceptors prolong the useable life of crucibles, classical split susceptor designs do not eliminate deformation problems from crucibles. Further, the susceptor design can influence the progression of crucible deformation. For instance, when the susceptor 9 shown in FIGS. 1 and 2 supports a vitreous silica crucible 21 in a high-temperature environment, the thermal expansion causes the supports 13 to open, as shown in FIG. 1. The retainer 17 holds the lower ends of the supports 13 relatively closer together while the susceptor splits 19 open wider near the top of the crucible 21. FIGS. 1 and 2 show exaggerated opening of the splits 19 near the rim of the crucible 21. Because the splits 19 allow the crucible 21 to expand more near its rim than near its bottom, the resulting stresses and elastic memory of the vitreous silica body of the crucible 21 tend to cause parts of the crucible sidewall 25 adjacent to the splits 19 and near the top of the crucible 21 to deform inwardly. Parts of the crucible sidewall 27 at the midpoints between the susceptor splits 19 on the circumference of the crucible 21 and near the top of the crucible tend to deform outwardly. Thus, after being subjected to high temperatures, the top of the crucible 21 tends to deform into an oval shape as shown in FIG. 2 in which the deformation has been exaggerated for clarity.
Deformation of the crucible can bring the crystal growing process to a premature end because it can prevent the crucible from being raised. For example, FIG. 3 shows a susceptor 31 supporting a crucible 33 inside a Czochralski crystal puller 35. The crystal puller 35 has a heat shield assembly 37 to shield the growing ingot 39 from heat and help it cool. As shown in FIG. 3, the crucible 33 is very near the upper limit of its vertical travel in the crystal puller 35. Some clearance 41 (not to scale) is provided between the crucible 33 and the heat shield assembly 37 to allow the rim of the crucible to be raised above the bottom of the heat shield assembly 37. Deformation of the crucible 33 can eliminate the clearance 41, making it impossible to raise the crucible further without striking the heat shield assembly 37. Thus, limited clearance between the crucible and other parts of the ingot growing apparatus, as exemplified by the system shown in FIG. 3, is one limit on the tolerance for crucible deformation.
Deformed crucibles also increase the risk of melt contamination. Deformation of the crucible can create pockets between the crucible and susceptor in which SiO from the crucible can react with the graphite susceptor to produce CO gas. The CO gas can react with the melt, resulting in SiC particulate formation in the melt. There is also an increased risk that particulate matter from the sidewall of a deformed crucible will fall into or otherwise contaminate the melt. Contamination of the melt results in the ingot having dislocations, impurities, or other defects. Thus, tolerance for crucible deformation is also limited by quality control factors.
Vitreous crucibles can be strengthened by applying a devitrification promoter to the inner and/or outer surfaces of the crucible sidewall. At high temperatures the devitrification promoter induces the coated surface to crystalize, thereby becoming cristobalite rather than vitreous silica. Because the devitrified portion of the crucible has more rigidity than the vitreous silica, the devitrification promoter coating results in the heated crucible sidewall having enhanced stiffness. Thus, continuous devitrification promoter coatings have been applied to the inner and/or outer surfaces of crucible sidewalls to make the crucible more resistant to plastic deformation. A variety of devitrification promoters, including calcium, barium, and strontium, are well-known to those skilled in the art.
Although the prior art crucibles having coated sidewalls are more resistant to buckling, their surfaces unfortunately tend to develop cracks in the devitrified layer. These cracks can be deep and stress induced. The devitrification induced by the devitrification promoter coating generally does not extend through the entire sidewall. Instead the coating results in a devitrified shell overlying vitreous silica. The tendency of the vitreous part of the sidewall to flow causes stresses in the devitrified shell. Further, the cristobalite and vitreous silica have different coefficients of thermal expansion. The mismatched expansion and/or stresses can cause a crack to form on the sidewall. If a crack forms on the inner surface, it is a potential source of particulates in the melt, which can result in defects in the crystal ingot. If a crack forms on the outer surface, there is a risk that the molten semiconductor material will dissolve through the now thinner sidewall and leak out of the crucible. Thus, cracks on a crucible can also bring a crystal growing process to a premature end.
FIGS. 4-5 show the same susceptor 9 discussed in connection with FIGS. 1-2, holding a crucible 51. The supports 13 are broken partially away at the split 19 in FIG. 4 to reveal more of the crucible. A devitrified shell 53 has been formed on the outer surface of the sidewall by application of a continuous coating of a devitrification promoter to the outer surface of the crucible sidewall. Because of the devitrified shell 53, the crucible 51 maintains its shape (FIG. 5) throughout a crystal growing process. Unfortunately, cracks 55 have formed on the crucible 51 sidewall adjacent to the midpoints 57 between the susceptor splits 19 on the circumference of the crucible. This is a typical pattern for crack formation.